1. Field of the Invention
The invention concerns a photosensitive device of the solid state type in which a signal amplification is achieved at the level of the photosensitive dots. The invention concerns, in particular, a new arrangement of the photosensitive dots, making it possible both to increase the operating qualities of these photosensitive dots and to broaden the scope of their use.
2. Description of the Prior Art
It is common practice to use photosensitive cells in solid state photosensitive devices wherein these photosensitive cells are placed so as to form a linear detector or array or else are placed in a matrix arrangement and form a photosensitive matrix. A photosensitive matrix has a system of conductors in rows and a system of conductors in columns. At each intersection between a row conductor and a column conductor, there is a photosensitive assembly or photosensitive cell hereinafter called a photosensitive dot. The photosensitive dots are thus organized in both rows and columns. According to a standard configuration, each photosensitive dot is connected between a row conductor and a column conductor.
Each photosensitive dot has at least one photosensitive element such as, for example, a photodiode or phototransistor, sensitive to the visible or near visible light photons. The light photons are converted into electrical charges which get collected in an electrical capacitor. This capacitor forms a storage capacity which may be formed either by the capacitor of the photosensitive dot itself or by an associated ancillary capacitor. A reading device can be used to interrogate the electrical state of the storage capacity, and to convey the electrical charge forming the signal towards a signal amplifier.
In the most standard configuration, a photosensitive dot is formed by a photosensitive element, a photodiode for example, series mounted with a transistor of the MOS type for example which fulfils solely a switch function.
FIG. 1 shows the drawing of a dot of a matrix of photosensitive dots of the prior act. This matrix has a number of rows and columns, and each photosensitive dot Pij is localized at the intersection of an order i row Li and an order j column Cj.
The row Li takes the form of a horizontally oriented row conductor, and the column Cj takes the form of a vertically oriented conductor Cj. The photosensitive dot has a photodiode Dij which is connected to the column conductor by a transistor Tij of the MOS type for example: in this example, the cathode of the photodiode is connected to the ground, and its anode is connected to the drain of the transistor, the source of which is connected to the column conductor, the gate of the transistor being connected to the row conductor. The light is picked up by the photodiode which is periodically pre-charged, and read by means of the transistor. The selection or addressing of the selected row is done by a pulse applied to this row and, hence, to the gate of the transistor so as to make it "on". Subsequently, the signal flows (in the form of charges) in the column conductor, and this signal is integrated in an integrating amplifier Aj.
With a view to simplifying the fabrication of solid state photosensitive matrices, a known way to fulfil the above-mentioned switch function is to use a diode that is series mounted with the photosensitive element as described, notably, in a French patent application No. 86 14058, publication No. 2 605 166, filed on 9th Oct. 1986 on behalf of THOMAS-CSF. This patent application describes solid state photosensitive devices wherein each photosensitive dot is formed by two diodes, mounted in series and upside down with respect to each other, i.e. with opposite directions of conduction. A first diode is connected to a row conductor, and the second diode is connected to a column conductor. This patent application further describes a method for the fabrication of this photosensitive device and a reading method that enables one of the two diodes to work as a "closed" switch when the row to be read is addressed, and thus enables the passage of the electrical charge forming the signal towards the signal amplifier.
One of the main problems raised by the reading of these types of photosensitive dots lies in an excessively great value of their electrical capacitance. This capacitance of each photosensitive element is applied to the column conductor and to the reading amplifier to which this element is connected, and tends to cause deterioration in the signal-to-noise ratio.
Another drawback displayed also by photosensitive dots of this type is known as "smearing", that is the cross-interference that exists between the signal coming from the photosensitive dot connected to a given column electrode, and the signals of all the other photosensitive dots connected to the same column electrode.
To improve the performance characteristics of these photosensitive devices, new structures of the photosensitive dot has recently been proposed, wherein the photocharges collected are not read directly but modulate a voltage or current. It is assumed, in this case, that there is an amplification of the signal at the level of the photosensitive dot itself.
As embodiment of this latter type of photosensitive dot is known from the IEEE publication "Transactions on Electron Devices", vol. 35, No. 5, May 1988, pp. 646 to 652. In this embodiment, the detection and the amplification are provided by a junction field effect transistor (J FET).
Another embodiment is known from the IEDM publication "Tech. Dig.", December 1986, pp. 353 to 356. The assembly described in this publication uses a transistor that is common to the amplification function and to the detection function. This transistor is of a type similar to a MOS transistor although, with respect to this transistor, it has differences made to it so that it can, notably, fulfil a detector function.
These two embodiments have, inter alia, the following defects:
(a) for the embodiment using a J FET transistor:
the technology of the J FET is relatively complicated and, therefore, expensive. It cannot be implemented in the context of a thin film type of technology.
it calls for the use of monocrystalline silicon so that, with respect to assemblies enabling the use of amorphous silicon, an embodiment of photosensitive dots such as this further has the drawback wherein it is not possible to make a large-area photosensitive device that can be used notably for the detection of X-ray images where it is particularly useful to have a large-area surface-image detector, 40 cm by 40 cm for example (it is known that for the detection of radiological images, it is enough to add a scintillator screen to a photosensitive matrix to convert X-rays into photons).
(b) For the second embodiment, which uses a transistor of a type resembling a MOS transistor:
this embodiment has the same defects as the above-mentioned ones, namely: complicated technology, the obligatory use of monocrystalline silicon which leads to the same drawbacks as the above-mentioned ones. And it would appear, furthermore, that this second embodiment has the drawback of not being protected against the smearing effect.